NEW YORK (CNNMoney.com) -- Users spend more time on Facebook than any other social network site. Much more. But other sites are growing quickly, and experts say no social network is safely on top of the market.

According to a report released Tuesday by Nielsen, Facebook users logged 13.9 billion minutes on the site in April. That compares to 5 billion minutes on MySpace, 300 million on Twitter, and 202.4 million minutes on LinkedIn.

Time spent on Facebook soared 699% since April 2008, compared to a 31% drop in time spent on MySpace, which is owned by media mogul Rupert Murchoch's NewsCorp (NWS, Fortune 500). LinkedIn's minutes grew 69%.

Changing of the guard: MySpace was once the dominant social network. In April 2008, 73% of the total time spent on social networks was spent on MySpace, according to Nielsen.

The scene has rapidly changed in the past year. Social network users spent just 23% of their time on MySpace in April 2009, compared to nearly 66% for Facebook.

"MySpace is not firing on all cylinders like Facebook," said Ray Valdes, social networking expert at Gartner. "Their effort to improve the site seems to be fragmented. They need to get on track or they will continue to slide."

Still, most users spend their time on Facebook on MySpace. The third largest social networking company is Google's Blogger, which with just 3% of the total time is way behind. Users spent 1.4% of their time on Twitter and 1% on LinkedIn.

April marked the fourth consecutive month in which Facebook held the most unique visitors and total minutes of any social network.

"Facebook continues to improve their user experience on their site, adding capabilities," said Valdes. "They're executing well, and it's showing up in their numbers."

A fickle audience: But the fastest-growing social network, by far, is Twitter, according to the report. Time spent on the micro-blogging Web site soared a whopping 3712% since April of last year. That soundly beat Tagged, which saw its minutes grow 998%. Facebook's year-over-year growth was third on the list.

Jon Gibs, vice president of Nielsen Online, said Twitter's 140-character-per-blog limit could change the landscape for social networking. But he warned that users are fickle, and it is too soon to call Facebook and Twitter's strong growth as a long-term trend.

"The one thing that is clear about social networking is that regardless of how fast a site is growing or how big it is, it can quickly fall out of favor with consumers," Gibs said in a statement. "Neither Facebook nor Twitter are immune."

MySpace remains the leader in online video streams, with 120.8 million in April, compared to 41.5 million on Facebook. Users spent 384 million minutes viewing videos on MySpace, or 38.8 minutes per viewer, according to the report. That trumped Facebook's 113.5 million minutes, or 11.2 minutes per viewer.